Semiconductor chips used are in a wide variety of microelectronic devices and are typically manufactured on a semiconductor wafer. The semiconductor wafer typically undergoes a wide variety of sequential processing steps during fabrication. At many points during the fabrication process, it is desirable to precisely align the semiconductor wafer relative to the tool performing the processes, to ensure that the processes are performed on the correct portion of the wafer. To aid in establishing the proper alignment between the wafer and the tool, semiconductor wafers accordingly include alignment markers. The alignment markers generally include a series of scribe lines or other features that produce an interference pattern when illuminated with radiation at a selected wavelength. The position of the wafer can be determined and adjusted as necessary by detecting the interference pattern produced by the radiation reflected from the alignment markers.
FIG. 1 is a partially schematic illustration of an alignment marker 10 configured in accordance with the prior art. The alignment marker 10 includes a first structure 20 and a second structure 30. The first structure 20 includes first features 21 and second features 22 interposed between neighboring first features 21. The first and second features 21, 22 are arranged in a repeating pattern having a pitch P1. The second structure 30 includes first features 31 and second features 32 interposed between neighboring first features 31, with the first and second features 31, 32 arranged in a repeating pattern having a second pitch P2. The first pitch P1 and the second pitch P2 are slightly different (e.g. by a factor of about 10%) so as to produce a combined interference pattern that is suitable for coarse alignment. Accordingly, the combined interference pattern produced by the first structure 20 and the second structure 30 can provide for coarse alignment, and the interference pattern produced by the features of either the first structure 20 or the second structure 30 alone can be used for fine alignment.
The diffraction patterns produced by the alignment marker 10 shown in FIG. 1 have multiple diffraction orders (e.g. first order, second order, third order, etc.). One drawback with the foregoing approach is that much of the radiant energy in the interference pattern may be focused in the first order. However, the first order is typically located at the center of the diffraction pattern and is therefore not very sensitive to spatial variations. Accordingly, it is desirable to increase the resolution obtained from the diffraction patterns by directing more energy to the higher diffraction orders than to the lower diffraction orders.
FIGS. 2A-2D illustrate alignment markers 10a-10d, respectively, configured to focus more energy in higher diffraction orders. For example, FIG. 2A illustrates an alignment marker 10a that replaces a single first feature within a given pitch P1 with two first features 21, and an intermediate second feature 22, so as to emphasize the third diffraction order. Each of the first features 21 and the second feature 22 have an equal width and are centered in the region normally occupied by the first feature 21 shown in FIG. 1. In FIG. 2B, a similar approach is used for an alignment marker 10b to emphasize the fifth diffraction order. In particular, three first features 21 and two intermediate second features 22, each with equal widths, are positioned in the region normally occupied by the first feature 21 shown in FIG. 1. FIG. 2C illustrates a similar approach for enhancing the seventh order. Specifically, the alignment marker 10c has four first features 21 and three intermediate second features 22, each with equal widths, and positioned in the region normally occupied by the first feature 21 shown in FIG. 1.
The marks shown in FIGS. 2A-2C have produced wafer quality improvements relative to the marks shown in FIG. 1, but the need to provide spatial discrimination for smaller and smaller features demands still further enhanced resolution. FIG. 2D illustrates a single alignment marker 10d having first and second features 21, 22, each with an equal width, evenly distributed over the entire pitch P1, rather than just the central portion of the pitch P1. The alignment marker 10d is intended to enhance the concentration of radiation for the fifth diffraction order. However, the single structure of the alignment marker 10d is not suitable for both fine and coarse alignment. Accordingly, there remains a need for improved alignment features.